Add action to handle network not found for multisite

[rmccue]

In a typical WordPress multisite installation, wp-config.php will contain definitions for both DOMAIN_CURRENT_SITE and PATH_CURRENT_SITE. The current network global is then set up based on this, then the site is searched for using get_site_by_path.

If the site isn't found, the ms_site_not_found action is then fired, allowing plugins to handle this and display an appropriate error, or redirect to the signup page.

However! In a subdomain-based multi-network install, the site is searched for first based on the domain. If one is found, the network is set from that site's data; otherwise, the network is searched for independently.

If no network is found, ms_not_installed() is called, with no way to set a "fallback" network and no actions or filters. The net result of this is that for multi-network installations, handling unknown domains (say, showing an information page as part of a domain mapping tool) is impossible. This means that domains have to show the DB error page, when the error is actually that they haven't been mapped to a site yet.

This was referenced in #30003, to which @jeremyfelt responded:

This is still expected behavior, but WordPress needs either DOMAIN_CURRENT_SITE and PATH_CURRENT_SITE to be populated in wp-config.php, $current_site to be populated through sunrise.php, or the $current_site global to be filled in via the lookup process so that it knows what network to redirect to.

...which is great and all, but the issue is that there's no way to handle this error in a custom way similar to the site-not-found error.

There's two ways I can see to handle this:

1. Add an action after the site lookup process, to allow setting defaults on the globals or doing other actions.
2. Add a ms_network_not_found action into the if ( empty( $current_site ) ) branch.

No. 2 is the easy choice, and I'd recommend adding it even if no. 1 is added. However, the first would help out during the normal request process as well.

[rmccue]

Option 1: Add lookup hook

[rmccue]

Option 2: Add network-not-found hook to match site-not-found hook

[rmccue]

Options 1 & 2: Both hooks

[rmccue]

Various patches added.

[jeremyfelt]

Let's look at this for 4.3.

[jeremyfelt]

I'm liking option 2 for this. It does match the title of the ticket perfectly. :)

I do think option 1 has a place, but that needs some more thought on another ticket as it provides more access to modifying bootstrap without requiring a sunrise file.

[jeremyfelt]

In 33990:

Multisite: Add action to handle network not found

ms_network_not_found fires when the global $current_site has not been filled and ms_not_installed() is about to fire. It cannot be used to populate $current_site, but can be used to capture the request and redirect or present a custom error.

Props rmccue.
Fixes #31702.